It has long been a desire of those working in the paper pulp art to produce a pulp mill that does not in any way significantly pollute the environment. A number of proposals have been made for such a pulp mill in the past, but the desired goal has yet to be achieved. For example, a "closed mill" was constructed at Great Lakes Forest Products, Thunder Bay, Ontario, in the 1970s, but it was difficult to run the mill closed for extended periods of time as a result of corrosion problems in the recovery boiler, and elsewhere, due to chloride buildup. See "Bleaching in the Closed Cycle Mill at Great Lakes Forest Products Ltd." by Pattyson et al, Pulp & Paper Canada, Vol. 82, No. 6, pp. 113-122 (1981). In the Great Lakes mill, bleaching plant effluents were introduced directly into the chemical recovery loop, as shown schematically in U.S. Pat. No. 4,039,372.
More recently, it has been proposed by HPD and Jaakko Poyry that closing of a pulp mill can be accomplished by evaporating acid effluent and then returning the E.sub.o bleach plant effluent to the brown stock washers. However that approach has yet to be successful, despite the utilization of plastic falling film evaporators which allow effective evaporation of the bleaching chemicals, and it is believed unlikely that it will ultimately be successful because of the buildup of undesired chemicals due to the introduction of the flow from the E.sub.o stage back to the brown stock washing stage.
According to the present invention, a method and apparatus are provided which utilize only existing technology, so that future development of sophisticated additional equipment or processes is not necessary, which essentially can reduce the liquid polluting effluents from a pulp mill to zero, provide only a minimum amount of solid waste for disposal (and provide the high probability that such solid waste can be used in an environmentally acceptable manner), and minimize the production of gaseous NO.sub.x and SO.sub.x products, so that the only significant gaseous pollutant from the pulp mill is carbon dioxide.
One of the basic aspects of the present invention that makes it possible to achieve these beneficial results is to treat the bleaching effluents completely separately from the chemical recovery loop until the effluents are in a particularly desirable form, and to then introduce the chemicals in that desirable form into the recovery loop. Another significant aspect of the present invention is the essentially complete oxidation of white liquor produced in the chemical recovery loop, which is then returned to the bleaching stage so that the proper balance between the various chemical treatment sequences is provided. Another significant aspect of the present invention that allows the desired results to be achieved are the production on site at the pulp mill, directly from the effluent streams and gaseous waste streams themselves, of essentially all of the sulfur dioxide, sulfuric acid, caustic or caustic substitute, and (if utilized) chlorine dioxide necessary to effect treatment of the pulp and recovery of the chemicals. Another factor which minimizes the amount of bleach plant effluents so as to make a proper treatment thereof practical, is advanced digesting techniques where delignification can be extended so that the pulp--without significant strength loss--discharged from the digesting stages has a low Kappa No. (e.g. 24 or below) and then the pulp is subjected to oxygen delignification to reduce the Kappa No. still further (e.g. to 14 or below, typically 10 or below) before bleaching is effected, allowing the production of prime market pulp (e.g. 88-90 ISO brightness).
The ability to produce prime market pulp with minimal adverse affect on the environment, according to the invention, is a quantum leap forward in pulping technology, and allows fulfillment of a long felt need to accomplish this desirable result.
According to one aspect of the present invention, a method of minimizing effluents from a cellulose pulp mill having a digester, bleach plant, and a recovery boiler and chemical recovery loop, is provided. The method comprises the following steps: (a) Concentrating (e.g. by evaporation) liquid effluents from the bleach plant to a concentration level high enough for incineration. (b) Incinerating the concentrated bleach plant effluents to produce a residue containing sodium, sulfate, carbonate, and sodium chloride. (c) Leaching the residue to produce a leachate. And, (d) feeding at least a substantial portion of the leachate to the chemical recovery loop associated with the recovery boiler.
The method also preferably comprises the further steps of: (e) Removing black liquor from the digester. (f) Increasing the solids concentration of the black liquor to a level high enough for incineration. (g) Incinerating the concentrated black liquor in the recovery boiler to produce a melt. (h) Producing white liquor and/or NaOH from materials in the recovery loop including the melt and the leachate fed to the recovery loop. (i) Oxidizing at least a part of the white liquor. And, (j) using at least a part of the oxidized white liquor in place of caustic in the bleach plant.
The invention also contemplates collecting spills of liquid from the pulp mill, evaporating the collected spills, and adding the concentrated spills to the concentrated bleach plant effluents in order to practice step (b). The spills are typically clarified before evaporation. There also are preferably the further steps of treating water removed from the bleach plant effluents by concentrating them, and then using the treated water as wash water in the bleach plant and in other mill processes.
Also there preferably are the further steps of producing substantially all caustic (or caustic substitute such as essentially completely oxidized white liquor) for the bleach plant, sulfuric acid, and sulfur dioxide needed for the plant processes, from process effluents and gaseous streams on site at the pulp mill so that no substantial external source of supply thereof need be provided.
Prior to feeding the leachate to the recovery loop, it is preferred that the leachate be crystallized and washed. The leachate also typically includes sodium chloride, and leachate containing chloride is used in the plant to produce substantially all of the chlorine dioxide necessary for the bleach plant. All of the metals above monovalent are removed from the leachate by washing, and those metals are kept out of the recovery loop and away from the bleach plant.
The bleach plant may have both acid and alkali liquid effluents, in which case it is desirable to initially evaporate (or otherwise concentrate) those different effluents separately, and then combine them for a final evaporation (concentration) before incineration. One typical bleaching sequence for the bleach plant may be DE.sub.o PD.sub.n D (where .sub.n refers to a neutralization stage between the two chlorine dioxide stages), and another typical bleaching sequence is AZE.sub.o PZP, although a wide variety of other bleaching sequences may also be utilized.
The invention also contemplates a method of recovering chemicals from bleach plant liquid effluents resulting from the production of chemical cellulose pulp by the following steps: (a) Concentrating (e.g. evaporating) the bleach plant liquid effluents to produce a concentrated effluent. (b) Incinerating the concentrated effluent to produce a residue. (c) Acting on the residue to recover sodium, sulfate, carbonate and/or sodium chloride. And, (d) using the recovered sodium, NaCl, sulfate and/or carbonate in the production of the chemical cellulose pulp.
The invention also contemplates a method of producing cellulose chemical pulp in a pulp mill, which requires sulfur dioxide, sulfuric acid, and caustic, and which has process effluents and gaseous streams, comprising the step of producing all of the sulfuric acid, sulfur dioxide, and caustic (or caustic substitute) necessary to effectively produce chemical pulp directly at the pulp mill, from the process effluents and gas streams, so that substantially no additional sulfuric acid, sulfur dioxide, or caustic is necessary from external sources.
According to another aspect of the present invention, apparatus for producing chemical pulp with a minimum discharge of effluents is provided. The apparatus comprises: A digester. A chemical recovery loop operatively connected to the digester, and including a recovery boiler. A bleach plant including at least one liquid effluent line therefrom. Concentrating means (e.g. evaporators) connected to the liquid effluent line from the bleach plant to produce a concentrated effluent. An incinerator for incinerating the concentrated effluent from the evaporator means, for producing a residue. And, means for recovering sodium, NaCl, carbonate and/or sulfate from the incinerator residue and feeding at least some of those recovered materials to the recovery loop. Also, water is recovered from the bleach plant effluents, which is used elsewhere in the mill.
The evaporator means preferably comprise a plurality of stages of metal-plastic laminate, falling film evaporators. Such evaporators are available from A. Ahlstrom Corporation of Helsinki, Finland, and Ahlstrom Recovery Inc. of Roswell, Ga. under the trademark "Zedivap", and described in co-pending application Ser. No. 07/974,060 filed Nov. 12, 1992 (corresponding to Finnish Application 915424 filed Nov. 18, 1991, and the disclosure of which is incorporated by reference herein). Although other evaporators, such as desalination evaporators, also are feasible, the "Zedivap".TM. evaporators are particularly advantageous and make the evaporating process for the bleach plant effluents practical. The evaporator means also may further comprise a concentrator between the stages of metal-plastic laminate evaporators and the incinerator.
According to yet another aspect of the present invention, the following apparatus is provided: A bleach plant for bleaching cellulose chemical pulp, and producing liquid effluents during bleaching. Means for concentrating (e.g. evaporating) the bleach plant liquid effluents to produce a concentrated effluent. An incinerator for incinerating the concentrated effluent to produce a residue. Means for acting on the residue to recover sodium, sulfate, NaCl, and/or carbonate. And, means for using the recovered sodium, sulfate, NaCl, and/or carbonate in the production of the chemical cellulose pulp being bleached.
The invention also contemplates the following apparatus: Means for acting upon all liquid effluents in the pulp mill so that no liquid effluents are discharged from the pulp mill to the environment. And, means for acting on all gaseous effluents from the pulp mill so that the amount of SO.sub.x and No.sub.x are minimized, and the only major adverse gaseous effluent is carbon dioxide.
According to still another aspect of the present invention there is provided the method of: Digesting comminuted cellulosic fibrous material to a Kappa No. of about 24 or below. Effecting oxygen delignification of the digested pulp to a Kappa No. of about 14 or below. Bleaching the oxygen delignified pulp to produce bleach liquid effluents. Concentrating (e.g. evaporating) the liquid bleach effluents into a concentrated effluent. Incinerating the concentrated effluent to produce a residue. And, acting on the residue to recover chemicals therefrom used in the digesting, oxygen delignification, and/or bleaching stages, while also recovering water.
As alternative to the procedures described above, in mills where a chemical plant for producing chlorine dioxide is provided for the bleach plant, the method of minimizing effluents may entail the following steps: (a) Concentrating liquid effluents from the bleach plant to a concentration level high enough for incineration. (b) Incinerating the concentrated bleach plant effluents to produce a residue containing sodium, sulfate, sodium chloride, and carbonate. (c) Distilling the residue from step (b) with sulfuric acid to produce gaseous hydrogen chloride, and remaining residue. (d) Using the gaseous hydrogen chloride from step (c) in the chemical plant for producing chlorine dioxide. And, (e) passing the remaining residue from step (c) to the recovery loop.
Preferably the recovery loop includes a recovery boiler, and step (e) is practiced to pass the remaining residue either directly to the recovery boiler, or first to evaporators and then to the recovery boiler. The remaining residue also may be treated to remove heavy and transition metals prior to the practice of step (e). Also the mill typically has a non-condensable gas system, including hydrogen sulfide and methyl mercaptan gases therein, and there are further steps of: (f) combusting the non-condensible gases in the non-condensible gas system to produce gaseous sulfur dioxide; (g) converting the gaseous sulfur dioxide to sulfuric acid; and (h) using the sulfuric acid from step (g) in step (c). Step (g) is preferably practiced by the Wet Sulfuric Acid (WSA) process. There may also be the further step (i), between steps (f) and (g), of reacting the gaseous sulfur dioxide from step (f) with sodium sulfite and water to produce concentrated sodium bisulfite and heating the concentrated sodium bisulfite in the presence of a catalyst to regenerate a concentrated form of gaseous sulfur dioxide; and wherein the concentrated form of gaseous sulfur dioxide from step (i) is used in step (g). Step (i) is known as the RESOX process.
Step (a) is typically practiced by evaporating the liquid effluents from the bleach plant, utilizing a plurality of stages of metal-plastic laminate falling film evaporators as described above.
Where the cellulose pulp mill bleach plant is a non-chlorine plant, a method of minimizing effluents may comprise the following steps: (a) Concentrating liquid effluents from the non-chlorine bleach plant, having little or no chlorine therein, to a concentration level high enough for combustion in the recovery boiler. (b) Passing the concentrated non-chlorine bleach plant effluents directly to the recovery boiler. And, (c) combusting the concentrated non-chlorine bleach plant effluents in the recovery boiler.
In all the procedures described, it is highly desirable to properly manage the water (liquid streams), so that the cleanest liquid is reserved only for those processes where it is needed, and so that its use is minimized where the absolutely cleanest water is not needed. Typically the pulp mill has at least first, second, and third loops of liquid streams, the loops having significantly different contamination levels, the contamination level gradually increasing from the first loop to the third loop. Minimizing the use of least contaminated liquid is achieved by: (a) sensing the contamination level in at least the first and second loops; (b) when the contamination level in the first loop exceeds a predetermined level, discharging some of the contaminated liquid from the first loop to the second loop, and replacing it with less contaminated liquid; (c) when the contamination level in the second loop exceeds a predetermined level, discharging some of the contaminated liquid from the second loop to the third loop, and replacing it with less contaminated liquid; and (d) purifying the most contaminated liquid from the loops to produce the less contaminated liquid for addition to at least step (b).
The method of managing liquid streams as described above is not limited to the methods heretofore described, but may be practiced in any pulp mill. Typically the method is practiced by combining the most contaminated liquid with the liquid effluents from the bleach plant to produce a combined effluent stream, and effecting evaporation of the combined effluent stream to produce a clean liquid and a more concentrated contaminant stream, with the clean liquid used as the less contaminated liquid in the steps described, while the more concentrated contaminant stream is further treated to recover contaminants therefrom (e.g. by incinerating and recovering chemicals from the incineration residue, etc., as earlier described). The pulp mill also typically includes a fourth loop and the contamination level in the third loop is sensed and when it exceeds a predetermined level some of the contaminated liquid is discharged from the third loop to the fourth loop, and replaced with less contaminated liquid. Also the contamination level in the fourth loop is preferably sensed, and when it exceeds a predetermined level liquid is discharged from it for the purification step earlier described. Liquid spills that are recovered from the pulp mill may be introduced into the fourth loop. There is also preferably the step of cooling the liquid in the loops, and in at least some of the loops separating contaminants from the loops and passing those separated contaminants to a more contaminated loop. Some of the contaminated liquid from the first loop may be passed directly to the third loop.
The invention also contemplates apparatus for managing liquid streams in a cellulose pulp mill having a digester, a bleach plant, and at least first, second, and third loops of liquid streams. The loops having liquid streams therein of significantly different contamination levels, the contamination level gradually increasing from the first loop to the third loop. This water managing apparatus comprises means for sensing the contamination level in at least the first and second loops; valve means controlled by the sensing means for discharging some of the contaminated liquid from one loop to another, more contaminated, loop; and means for purifying contaminated liquid (typically the most contaminated) from the loops to produce less contaminated liquid. The apparatus may also include means for cooling the liquid in each of the loops. Also, at least one of the loops includes means for separating a rejects stream from the loop, and for discharging the reject stream to another, more contaminated, loop
It is the primary object of the present invention to provide for the production of cellulose chemical pulp with essentially zero discharge of liquid pollutants to the environment, with a minimum amount of gaseous pollution, and with the minimum amount of solid waste products. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.